And of the three, macOS has the least promising future (business/profit wise) of the three. There's plenty of room to grow iCloud services, plenty of more devices and device types you can put iOS on, but laptops and desktops can only grow so much in a post PC world.
I disagree. If you're looking for a platform that will last into the future, consider that Windows has survived for three decades now, and Unix for
five. People are still using these operating systems because, although the hardware has changed out of all recognition in the last quarter-century, the investment people have put into learning the system and the legacy applications they run are enticements for them to continue using the system.
Apple encourages similar behavior under iOS, but via a "walled garden" ecosystem. You don't stay inside Apple's ecosystem because you
want to, you do so because you
have to.
As such, Apple doesn't feel nearly as much pressure to get its ecosystem onto as broad a range of hardware as other companies do. Microsoft makes more money if they can get Windows running on absolutely
everything; but Apple makes more money if they can convince customers to live with fewer hardware choices. Fewer platforms means less development work, ability to buy parts in greater bulk (reducing costs), and decreased support problems.
The downside to this? iOS is
fragile. Placing all the iOS eggs in just a few baskets means that if you stumble and drop one of those baskets, you can lose a whole lot. If Apple ever faced an issue like Samsung did with the Note 7, they might lose a whole lot more than Samsung did; or even if some more gradual issue appeared that drove away customers, it'd cause a much bigger rupture for the company.
Because, if you get tired of a particular piece of hardware in the Windows world (or even the Android world), you can easily migrate to another manufacturer, but still drag along all your legacy software and all your knowledge about the OS with you. If you get tired of Apple's hardware? Well, that's it. You're gone completely, all your investment into iOS wasted.
And no, there
isn't a whole lot of room to grow iCloud services, at least not for the iPhone, and probably not for the iPad either. The smartphone is no longer an emerging market; everybody now has one (just like PCs!). You would need to create new devices to place iOS on, but the little-screen-in-your-hand market is now saturated, and iOS is designed
specifically for little-screen-in-you-hand devices. New iOS devices today are purchased mainly as replacements for older iOS devices; if Apple ever loses those customers, good luck dragging them back into the ecosystem.